Some discrete power semiconductor devices such as IGBTs (insulated gate bipolar transistors), MOSFETs (metal oxide semiconductor field effect transistors), JFETs (junction field effect transistors), power diodes, etc. include an integrated polysilicon diode as a temperature sensor. A known fixed current is driven through the polysilicon diode and the absolute forward voltage drop of the diode is measured. Ideally, the absolute forward voltage drop of the diode decreases linearly with temperature. Hence, the junction temperature of the diode can be directly concluded from the measured forward voltage drop using a known relationship between forward voltage drop and diode junction temperature. However, production variation inherent in semiconductor manufacturing causes a wide variation in the forward voltage behavior of polysilicon diodes. As a result, the accuracy of polysilicon diode based temperature sensors is relatively low.
In addition, the resistance of traces inside the semiconductor die (chip) from the external terminal to the polysilicon diode and back to the terminal increase the error. Current flowing through the diode creates not only a forward voltage drop across the pn junction of the diode, but also a voltage drop within the traces. Hence, the measured diode voltage is greater than the actual pn junction voltage. While the sign of the error is known, the absolute magnitude is not, further compounding the measurement error.
Furthermore, variations of the test current driven through the diode also introduce error. For example, if the test current increases e.g. due to temperature changes or lot-to-lot changes of that test circuit, then the forward voltage of the diodes increases and this is erroneously interpret as lower temperature. In view of the above and other considerations such as lower cost and complexity, a more accurate temperature sensor and temperature sensing technique is desired for discrete power semiconductors.